Doxazosin is chemically known as 4-amino-2-[4-(1,4-benzodioxan-2-carbonyl)piperazin-1-yl]-6,7-dimethoxyquinazoline and is represented by formula I.

Doxazosin, a quinazoline compound, is an alpha-1 adrenergic receptor blocker used to treat high blood pressure, benign prostatic hyperplasia and elevated serum low density lipoproteins. It inhibits the binding of norepinephrine to alpha receptors in the autonomic nervous system thus causing vasodilation, and a decrease in peripheral vascular resistance which further leads to decrease in blood pressure.
U.S. Pat. No. 4,188,390 discloses doxazosin, its pharmaceutically acceptable salts and process of preparation thereof. U.S. Pat. No. 4,188,390 covers the preparation of doxazosin hydrochloride only and not of the doxazosin base.
The doxazosin hydrochloride is prepared by reacting 4-amino-2-chloro-6,7-dimethoxy quinazoline with N-(1,4-benzodioxan-2-carbonyl)piperazine as represented in following reaction Scheme 1.

The major problem associated with the disclosed process is that reaction of piperazine with ethyl 2,3-dihydrobenzo[1,4]dioxin-2-carboxylate results in formation of the bis-amide impurity.
Organic Preparations and Procedures International (2003); 35; 603-608 describes the procedure for synthesizing doxazosin by treating (2,3-dihydrobenzo[b][1,4]dioxin-2-yl) (piperazin-1-yl)methanone with 2-chloro-6,7-dimethoxy quinazolin-4-amine in presence of n-butanol to obtain doxazosin hydrochloride; which on further treatment with aqueous ammonia solution yields doxazosin base.
However, the process described in Organic Preparations and Procedures International (2003) involves the use of highly toxic reagents like thionyl chloride which is not convenient to handle at industrial scale. Also, the use of the reagent in the process leads to generation of unwanted side product such as dimeric impurity, formula A, which in turn affects the overall yield of the final product. Hence, the preparation of doxazosin hydrochloride by this process is not feasible at plant scale.

Other patents disclose preparation of doxazosin mesylate and its polymorphs.
From the prior art it is clear that there has been no disclosure of a process of preparing doxazosin. The known processes give the preparation of doxazosin salt which on treatment with a base yields doxazosin. The inventors have therefore felt a need to develop a process for the preparation of doxazosin that is simple, easy to scale up and industrially acceptable.